This invention relates to the inspection for defects arising during the fabrication of integrated circuits and, in particular, a method and an apparatus for scanning a light pattern to detect the presence of defects in a specimen subject to which the light pattern corresponds.
The use of light images in association with spatial filters has been suggested heretofore for determining the presence of defects in a photomask used in the fabrication of microminiature integrated circuits. One system that purports to accomplish this task is described in U.S. Pat. No. 4,000,949 of Watkins.
Light image constructed from holograms incorporated in optical processing apparatus are especially beneficial in that reverse ray tracing techniques can be employed during hologram reconstruction. Reverse ray tracing is advantageous in eliminating aberrations which are introduced by the optical components in the apparatus and which would spread out the light rays representing defects in a photomask pattern that was exposed to form the hologram. The spreading of light rays impairs the ability to detect the defects. A system employing reverse ray tracing techniques is described in U.S. Pat. No. 4,516,833 of Fusek.
The detection of defect image information in the light pattern can be accomplished in accordance with the following procedure. A light sensitive device having a defined imaging field is positioned to expose the device to and measure the intensity of light present in the defect image region of the size and shape of the imaging field and then is moved to the next adjacent region to repeat the measurement process. The problem with a detection procedure of this type is that relatively long settling times are required after movement of the light sensitive device from one measurement position to the next adjacent one. The reason is that the alignment position of the light sensitive device relative to the defect image region must be stable to ensure that detection of an image defect of submicron dimensions can be effected. A detection system of this type is, therefore, unacceptable in applications requiring relatively high throughput defect detection rates (i.e., the rate at which an array type photomask with normally identical elements can be inspected completely).